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{\title Articles for}
{\subject }
{\author Gene Fry}
{\keywords }}\vieww12240\viewh14820\viewkind1
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\f0\b\fs24 \cf0 Edges of the Tropics Push Toward Poles: Global Warming?\
\b0 by Gene Fry\
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\cf0 Feb. 2008 - Earth\'92s zone of tropical climate is advancing toward the poles, according to a study in December 2007\'92s
\i Nature Geoscience
\i0 , which summarizes four recent studies of atmospheric systems around the tropics. Climatologists define the tropics in several ways.\
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\cf0 Above all, the tropics are the regions where Hadley cells operate. Warm air rises, laden with moisture, near the equator. It then cools, shedding moisture as rainfall. At higher altitudes, the air flows north and south, away from the equator. The drier air sinks back to Earth at the edge of the tropics, roughly 1,500 miles (2,500 kilometers [km]) poleward. From there, the dry air moves along near the surface, generally back toward the equator. Thus, Earth is rainy near the equator, but drier near the fringes of the tropics, in places like the Sahara and Sonoran deserts.\
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\cf0 A second feature defining the tropics is the general east to west direction of the winds. In contrast, at mid-latitudes, large-scale winds move west to east. A
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\b0 3rd feature is a pair of jet streams (north and south) that blow at the boundary between the tropics and mid-latitudes. (Another pair blows at the boundary between the mid-latitudes and polar areas.) A 4th feature is the higher altitude of the tropopause over the tropics. The tropopause separates the troposphere (the lowest level of the atmosphere) from the overlying stratosphere. Finally, Hadley cells move stratospheric ozone from the tropics toward the poles.\
Computer simulations dating back to 2001 predicted that as Earth warmed, the Hadley cells and their associated wind and rainfall patterns would move poleward. Under the most extreme scenario, the tropics would expand, on average, by about 2\'b0 latitude (about 140 miles or 220 km) over the 21st century. The new study looks at actual movement of these 5 features from 1979 to 2005. The review concludes that, \'93Remarkably, the tropics appear to have already expanded
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\f0 during only the last few decades of the 20th century
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\f0 by at least the same margin as models predict for this [entire 21st] century.\'94 The 5 data sets, for the 5 features, showed expansion of 2\'b0 to 4.8\'b0 latitude over the 25 years, or 220\'96530 km (140\'96 330 miles). \'93[T]his change \'85 has already happened
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\f0 and the worst-case scenario has already been surpassed,\'94 the paper added. \'93The observed widening appears to have occurred faster than climate models predict in their projections of anthropogenic [manmade] climate change.\'94\
The paper raises profound questions. One is the accuracy of climate models that drive scientific conclusions about the pace of climate change. Another is the mechanisms that have caused the tropics to widen. Global warming is not the only possible explanation. However, recent computer models have consistently underestimated the ill effects of climate change, noted climate scientist Andrew Weaver of the University of Victoria (Australia). \'93Unexpectedly rapid expansion of the tropical belt constitutes yet another signal that climate change is occurring sooner than expected,\'94 said Professor Barry Brook of the University of Adelaide (Australia). \'93The case for rapid action on greenhouse gas [GHG] emissions becomes that much more compelling,\'94 Brook added.\
Widening of the tropics carries worldwide implications. The study notes that, \'93The edges of the tropical belt are the outer boundaries of the subtropical dry zones. \'85 [T]heir poleward shift could lead to fundamental shifts in ecosystems and in human settlements.\'94 Poleward expansion of the tropics is likely to bring drier conditions to heavily populated regions, even as it may bring more moisture to equatorial regions. Thus, poleward expansion of the tropics will intensify water scarcity in the Mediterranean, the US southwest, northern Mexico, southern Africa, southern Australia, and parts of South America, the study warns. \'93Shifts in precipitation would have obvious implications for agriculture and water resources and could present serious hardships in marginal areas.\'94 Weaver commented, \'93You\'92re not expanding the tropical jungles, what you\'92re expanding is the area of desertification.\'94 Expansion of tropical belts could also exacerbate climate change by increasing the rate at which water vapor, the leading GHG, is being pumped into the upper atmosphere.\
Highlights of the study are available at
\i http:// uk.news.yahoo.com/afp/20071202/tsc-scienceclimate- warming-tropics-c2ff8aa_1.html
\i0 and at
\i www. dailycomet.com/article/20071202/APA/712020641
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\b CONTACT:
\b0 Dian Seidel, US National Oceanic and Atmospheric Administration\'92s Air Resources Laboratory, Silver Spring, Maryland (USA). E-mail:
\i dian.seidel@noaa.gov
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